System for measuring partial discharges

ABSTRACT

A system for measuring partial discharges comprises a plurality of acquisition circuits each including a signal shaping circuit delivering a signal value, a multiplexer enabling the signal values provided by the various acquisition circuits to be delivered in succession on a common path, an acquisition trigger circuit responding to the existence of an acquisition command signal provided when the input level of at least one of the acquisition circuits exceeds a defined threshold and defining an acquisition command signal, and a signal treatment and storage system responding to the acquisition command signal and then performing an acquisition cycle during which it stores the values of the signals provided by the various acquisition circuits via the multiplexer.

The present invention relates to a system for measuring partialdischarges.

BACKGROUND OF THE INVENTION

For example, partial discharges occur in equipment used in the hightension installations of networks for transporting and distributingelectrical power. In a sequence of dielectrics subjected to hightension, one item is the seat of a breakdown, and the energycorresponding to the capacitive charge of said item is dissipated in theform of high frequency oscillation. The other items in the sequenceretain their integrity, so the partial discharge does not lead tooverall breakdown.

A study of partial discharges can be used to evaluate the quality of apiece of equipment and to provide useful information when quality is tobe improved. It is therefore important to be able to performmeasurements relating to partial discharges. Special measuringapparatuses have been developed for this purpose. By way of example,mention may be made of measurement system 9100 provided by the firm"Tettex AG" and described in the publication "TettexInstruments-Information", No. 21, April 1987. Such apparatus performsmeasurement on current and voltage pulses produced by partial dischargesand it displays the maximum values observed. Individual pulses may alsobe observed by means of an oscilloscope. In addition, the repetition ofpartial discharges is also significant. In communication No. 15-12,entitled (in translation) "Search for correlation between the energy ofpartial discharges and the degradation of paper/oil insulation", by F.Viale et al., given to the 1982 session of the International Conferenceon Large Electricity Networks, mention is made of integrating the energyof partial discharges over a given period.

The article "Simultane Erfassung und Verarbeitung vonTeilentladungs-Kenngrossen zur Beurteilung elektrischer Isolierungen" byF. H. Kreuger et al., describes a system in which a microcomputerreceives various digital values relating to each instant of a singlepartial discharge. The article deals with measures to be taken forreducing the volume of information to be stored and it even mentionsconserving the time conditions of the recorded signals, but withoutproposing any solution.

Patent document DE-36 12 234 describes a system of the same type.

U.S. Pat. No. 4,757,263 describes apparatus for monitoring variouselectrical magnitudes in a single installation, however these magnitudesare basically continuous in nature, unlike the signals generated bypartial discharges which are pulse signals and unpredictable.

Observing partial discharges at a plurality of points gives rise to theproblem of recording a huge quantity of information continuously orquasi-continuously, unless observation periods are selected arbitrarily,in which case there is a danger of missing useful observations, orunless the results of the observations are treated which is expensive intreatment means. None of these solutions is satisfactory. In practicethis prevents an experimenter from having a complete picture ofdischarge activities, and leaves it to the experimenter's experience orskill in selecting discharges that are particularly significant.However, there are numerous cases when it would be desirable to obtainfuller information on partial discharges, particularly during studiesbearing on the correlation between the discharges and the deteriorationof the equipment in which they take place. Although the energy of apartial discharge is indeed a measure of its harmfulness, it is alsonecessary to know what type of discharge is involved, i.e. where it hastaken place. This may be deduced from observations made simultaneouslyat several points, for example.

A particular object of the present invention is to provide a system formeasuring partial discharges and satisfying this requirement.

SUMMARY OF THE INVENTION

The present invention provides a measurement system for measuringpartial discharges comprises a plurality of acquisition circuits, eachincluding a signal input and a signal shaping circuit for providing asignal value, a multiplexer serving to deliver signal values provided bythe various acquisition circuits, said signal values being delivered insuccession on a common path leading to a signal treatment and storagesystem, wherein some of the acquisition circuits are partial dischargeacquisition circuits and receive different partial discharge signalscoming from different points of an installation, and wherein anacquisition triggering circuit is provided which responds to theexistence of a partial discharge signal as provided by a comparatorwhenever the input level to at least one of said partial dischargeacquisition circuits exceeds a defined threshold by delivering anacquisition command signal, said signal treatment and storage systemresponding to an acquisition command signal by performing an acquisitioncycle during which it stores the values of the signals provided by thevarious acquisition circuits via the multiplexer.

Such a system makes it possible to record groups of signal valuesasynchronously, each signal value corresponding to one of successivepartial discharges. However, it is important to locate the partialdischarges in time.

One of said acquisition circuits is a voltage acquisition circuit and itprovides an instantaneous measurement of the alternating voltage of thedistribution network.

This makes it possible to locate a group of signal values relative tothe half-period of the alternating voltage.

The voltage acquisition circuit may include means making its supply of ameasurement of the alternating voltage of the distribution networkconditional on the presence of said acquisition command signal.

Said acquisition trigger circuit may include means that respond to theappearance of a maximum voltage value on the input of said voltageacquisition circuit by providing an additional acquisition commandsignal, the said signal processing and storage system responding to theadditional acquisition command signal by performing an acquisitioncycle.

This has the consequence of making it possible to locate groups ofrecorded values relative to the half-periods of the network voltage.

In one embodiment, said signal treatment and storage system is asuitably programmed commercial computer. The signal values are convertedinto digital form, preferably after being multiplexed.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the invention is described by way of example withreference to the accompanying drawings, in which:

FIG. 1 is a theoretical diagram of circuits for obtaining signals to bemeasured by the system of the invention;

FIG. 2 is a connection diagram of the system of the invention when themeasurements relate to a transformer;

FIG. 3 is a block diagram of a circuit for acquiring partial discharges;

FIG. 4 is a block diagram of a circuit for acquiring the alternatingpower supply voltage;

FIG. 5 is a diagram of an acquisition control circuit; and

FIG. 6 is an overall diagram of a partial discharge measuring system inaccordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference is made initially to FIG. 1 which shows test equipment Zconnected between ground and a high tension power line HT together withboth a capacitive voltage divider constituted by capacitors C1 and C2for obtaining voltage measurement signals, with output st taken from thecommon point between the capacitors making a voltage signal available inthe form of an alternating voltage whose value is a fraction of the hightension, and also, in conventional manner, a capacitor C3 and animpedance zm constituting a voltage divider for obtaining partialdischarge measurement signals, with the output sd taken from the commonpoint between them delivering a partial discharge pulse signal each timea partial discharge occurs in the equipment Z, which impulse signal hasthe general form of a damped sinewave oscillation.

As an example of equipment under test, FIG. 2 shows a three-phasetransformer TR having three primary windings ep1, ep2, ep3 in a deltaconfiguration connected to the three wires a, b, and c of a low tensionline, and having three secondary windings es1, es2, es3 in a starconfiguration connected to the four wires A, B, C, and N of adistribution line.

In accordance with the circuit shown in FIG. 1, capacitors c3p1, c3p2,c3p3, c3s1, c3s2, c3s3 and c3n are provided to perform the measurementfunction of capacitor C3, and impedances zmp1, . . . , zms3, and zmn areprovided to perform the function of the impedance zm, relative topartial discharge output signals sdp1 to sds3 and sdn. Similarly,capacitors c1p and c2p perform the function of capacitors C1 and C2relative to a voltage signal output.

The partial discharge acquisition circuit MAD of FIG. 3 treats any oneof the partial discharge signals and shapes it. In addition, itcontributes to generating an acquisition command signal. This circuithas an input EN connected for example to output sds1 of FIG. 2 and itincludes a first inverting amplifier ai1 for matching levels. Theresulting signal at the output from amplifier ai1 is applied to one ofthe inputs of each of two comparators ce1 and ce2 which also receive tworeference voltages V+ and V-. If the output signal from the amplifierai1 exceeds the voltage V+, then the comparator ce1 provides a signalwhich passes through gate pt1 and changes the position of bistable bs1.This then provides a signal vd1 which activates a peak detector DC1.Simultaneously, a complementary signal vr1 blocks the gate pt2.Thereafter, the peak detector DC1 which receives the output signal fromamplifier ai1 via an additional unit-gain inverting amplifier ai2maintains a signal level on its output sd1 corresponding to the maximumpositive level reached by the input signal. Simultaneously, the outputprovides a signal DP via an OR gate po indicating that a partialdischarge has been detected. The level reached by the partial dischargeis read by delivering a signal AC. Together with the signal vd1, thesignal AC unblocks the gate pt3 and causes a switch cm1 to operate,closing a contact ct1, such that the level of the signal present on theoutput sd1 of the peak detector DC1 is delivered to the output ST of thecircuit.

If the input signal is negative and exceeds the threshold V-, thenoperation via the comparator ce2, gate pt2, bistable bs2, gate pt4, andswitch cm2 closing contact ct2 is the same, and peak detector DC2receives the output signal from the amplifier ail directly andconsequently treats a signal having the same polarity as the peakdetector DC1, while the unity gain auxiliary amplifier ai3 inverts theoutput signal sd2 from the peak detector DC2 to provide a signal sd2' onthe output ST and having the same polarity as the input signal. Thesignal DP is provided by the gate po as before.

In this way, the circuit MAD of FIG. 3 selects the polarity of the firsthalf cycle of a partial discharge signal when one appears, and itretains the value of the peak voltage reached for this polarity, whichvalue is prepared as the value of the signal, and in addition a signalindicative of the presence of a partial discharge is provided to requestacquisition. Acquisition serves to read the value of the signal onoutput ST. Thereafter a signal RZ resets the bistable bs1 or bs2 and thepeak detector DC1 or DC2 to zero and the circuit returns to its initialstate.

The power supply alternating voltage acquisition circuit MAT of FIG. 4shapes the voltage signals coming, for example, from the output st ofthe circuit of FIG. 2 and triggers acquisition whenever the voltageexceeds a positive or negative maximum. In fact, these two aspects aretreated separately.

The voltage signal is shaped mainly by means of a sampling circuit ECHwhich receives the signal delivered to the input ENT of the circuit viaa unity gain impedance-lowering amplifier ae. The signal DP delivered byany of the circuits as shown in FIG. 3 when a partial discharge occurschanges the position of a bistable be which enables the sampling circuitECH. This records the value of the voltage being delivered to it at thisinstant by the amplifier ae and maintains the recorded value on itsoutput se. At the acquisition instant, the output signal from bistablebe enables the signal AC to pass through the gate pe and operate theswitch ce which then closes its contacts ce1, thus delivering the valueof the sampled voltage on output STT.

Thus, this portion VT of the circuit shown in FIG. 4 operates as acomplement to the circuits MAD such as that shown in FIG. 3, and on theoccasion of each partial discharge it provides a value of thealternating voltage read from one of the high tension line conductorsfeeding the equipment under test; this makes it possible to locate thepartial discharge within a half period of the alternating voltage.

The invention also provides for detecting and recording the positive andnegative maxima of the alternating voltage in order to enable partialdischarges to be located in time relative to half-periods of thealternating voltage.

To this end, the input voltage to the circuit MAT of FIG. 4 is appliedvia a unity gain impedance reducing amplifier at1 to a comparator cmt1which also receives a voltage Vr+. This voltage is relatively low andserves to direct the positive and negative half cycles of thealternating voltage to circuits designed to treat them. When thisvoltage is exceeded, the comparator cmt1 provides a signal ve1 whichenables the maximum detector circuit DM1. A circuit of this type is soldby "Precision Monolithics Inc.", under reference PKD-01. This circuitmonitors the signal which is delivered thereto by the amplifier at1, andwith appropriate time constants, and delivers a signal mx1 when theamplitude of this signal begins to drop. Via a gate pm, this signal mx1provides a signal MAX indicative of a maximum (positive in this case) inthe evaluated alternating voltage and requests an acquisition. Inaddition, the circuit DM1 samples the alternating signal and maintainson its output sm1 the sample which it obtained at the moment when itdelivered the signal mx1. Via the gate px1, the signal mx1 enables thegate py1. During acquisition, a signal CR is delivered to the circuit ofFIG. 4 and, via the gate py1, it actuates the switch cx1 which closesits contacts tx1. The positive maximum value of the alternating voltageis thus transferred to the output STT.

The input voltage reproduced at the output of the amplifier at1 is alsoapplied to a comparator cmt2 for detecting when a threshold Vr- ispassed and consequently delivering a signal ve2 enabling the circuit DM2which is identical to the circuit DM1. This circuit DM2 receives theinput voltage via a unity gain inverting amplifier at2. It thus operatesin the same way as the circuit DM1. It thus provided a signal mx2 whenthe input voltage passes through a negative maximum, whereupon the gatepm provided the acquisition request signal MAX. The sampled valuemaintained on the output sm2 of the circuit DM2 is inverted by invertingamplifier at3. On acquisition, since the gate px2 is enabled by thesignals ve2 and mx2, it enables the gate py2 which transmits the signalCR to the switch cx2 which closes the contacts tx2 and the negativemaximum value of the input voltage to the circuit of FIG. 4 istransmitted to its output STT.

The circuit MAT of FIG. 4 thus causes a maximum voltage to be acquiredduring each half cycle of the power supply voltage. By counting thesehalf-periods, it is possible to follow the passage of time until apartial discharge aquisition appears, thereby enabling this partialdischarge to be located in a defined half-period. It is recalled thatlocalization within the half-period will result from the voltage valuedelivered by the sampling circuit ECH in comparison with the values ofadjacent maximum voltages.

The acquisition control logic circuit LA of FIG. 5 serves as aninterface between the circuits MAD and MAT of FIGS. 3 and 4 and asuitably programmed general purpose computer ORD. If the bistable bc2 isindeed at rest, a signal DP from a circuit such as that shown in FIG. 2acts via a gate pc1 to change the position of a bistable bc1 whichblocks a gate pc2. The bistable bc1 delivers the above-mentioned signalAC by means of which each circuit MAD such as that shown in FIG. 3provides a partial discharge signal on its output ST corresponding tothe peak amplitude of the first half-cycle of the pulse signal caused bythe partial discharge, whereas the circuit MAT of FIG. 4 provides thevalue of the alternating voltage at said instant as maintained by thesampling circuit ECH.

The signal AC is also transmitted to the computer ORD in order to causean acquisition cycle to be executed, whereby the values presented inthis way are recorded. Thereafter, after an adequate delay provided by adelay stage ER, the circuit LA provides the signal RZ which resets thebistable bc1 and simultaneously resets the circuits MAD and MAT of FIGS.3 and 4.

When the signal MAX is delivered by the circuit of FIG. 4, and if or assoon as the bistable bc1 is at rest, gate pc2 conducts and bistable bc2changes position, thereby closing gate pc1 and providing the signal CR.An acquisition cycle similar to the preceding is accomplished. Itterminates by delivering a signal RZ' via the delay circuit ER' of themodule LA itself, thereby returning the bistable bc2 to its initialposition and also the maxima detector DM1 or DM2 of the circuit of FIG.4.

Finally, FIG. 6 is a diagram showing a computer ORD provided with aninterface INT containing a multiplexer MTX, and an analog-to-digitalconverter CAN. The acquisition circuits MAD and MAT of FIGS. 3 and 4 andthe acquisition command logic LA of FIG. 5 are connected to theinterface INT. The interface INT converts the signals CR and AC of thelogic circuit LA into interrupt requests in response to which thecomputer ORD addresses the circuits MAD/MAT via the multiplexer MTX andobtains digital values via the interposed converter CAN, which valuescorrespond to the analog magnitude available on the outputs ST and STT.The signals CR and AC may also give rise to identical acquisition cyclesin which, returning to the configuration shown in FIG. 2, eight pathstreated by seven circuits MAD and one MAT circuit are all concerned byone recording cycle. If the recording cycle is caused by a partialdischarge appearing, then the eight paths provide significant, i.e.non-zero, information. If the recording cycle is caused by establishinga maximum in the alternating power voltage, then the seven first pathsprovide zero values (since the signal AC is not present), and only thepath corresponding to the circuit MAT provides a non-zero signal(constituted by the maximum observed positive or negative amplitude ofthe alternating voltage). Such records are therefore easily recognizedfor the purpose of counting half-periods of the alternating voltage andthus of locating in time the records relating to partial discharges.

Naturally the above descriptions are given purely by way of non-limitingexample and numerous variants may be envisaged without thereby goingbeyond the scope of the present invention.

We claim:
 1. A measurement system for making measurements relating topartial discharges, comprising a plurality of acquisition circuits, eachincluding a signal input and a signal shaping circuit for providing asignal value, a multiplexer serving to deliver signal values provided bysaid various acquisition circuits, in succession, on a common pathleading to a signal treatment and storage system, wherein some of saidacquisition circuits are partial discharge acquisition circuits, each ofsaid partial discharge acquisition circuits being connected to one of aplurality of measurement points of an installation subject tomeasurement, each partial discharge acquisition circuit comprising acomparator for providing a partial discharge signal whenever a partialdischarge affects their signal input, and wherein an acquisitiontriggering circuit is provided which responds to the existence of apartial discharge signal as provided by any one of said partialdischarge acquistion circuits by delivering an acquisition commandsignal, said signal treatment and storage system responding to anacquisition command signal by performing one acquisition cycle duringwhich it stores the signal values provided by said various partialdischarge acquisition circuits via said multiplexer.
 2. A measurementsystem according to claim 1, wherein one of said acquisition circuits isa voltage acquisition circuit and provides an instantaneous measurementof the alternating power supply voltage of said installation.
 3. Ameasurement system according to claim 2, wherein said voltageacquisition circuit includes means for making its supply of ameasurement of the alternating power supply voltage of said installationconditional on the presence of said acquisition command signal.
 4. Ameasurement system according to claim 2, wherein said acquisitiontrigger circuit includes means for responding to the appearance of amaximum voltage value on the input of said voltage acquisition circuitby providing an additional command signal.
 5. A measurement systemaccording to claim 1, wherein said signal treatment and storage systemis a suitably programmed computer.
 6. A measurement system according toclaim 5, wherein the values of the signals are converted into digitalform between the multiplexer and said computer.